𝐼-centered versus 𝐹-centered orthorhombic symmetry and negative thermal expansion of the charge density wave of EuAl2⁢Ga2

Abstract

Together with EuGa4 and EuAl4, EuAl2⁢Ga2 belongs to the BaAl4 structure type with space group 𝐼⁢4/mmm. EuAl2⁢Ga2 develops an incommensurate charge density wave (CDW) at temperatures below 𝑇CDW=51K. On the basis of temperature-dependent single-crystal x-ray diffraction data, the incommensurately modulated CDW crystal structure of EuAl2⁢Ga2 is determined to possess orthorhombic symmetry Immm(0000γ)𝑠⁢0000. This symmetry is different from the orthorhombic Fmmm-based symmetry of the CDW state of EuAl4. Nevertheless, both symmetries, Immm(00γ)𝑠⁢00 and Fmmm(00γ)𝑠⁢00, lead to the same conclusion, that the CDW is supported by the layers of Al1 type atoms, while the Eu and Al2 or Ga atoms are not directly involved in CDW formation. The different symmetries of the CDW states of EuAl4 and EuAl2⁢Ga2, as well as the observation of negative thermal expansion in the CDW state of EuAl2⁢Ga2, might be explained by the effects of Ga substitution in the latter compound.